I. Field
The present invention relates generally to the removal and capture of acid gases, including carbon dioxide, from flue gas or other gases through aqueous absorption and stripping processes. More particularly, it provides methods for reducing the energy consumption of such absorption and stripping processes.
II. Description of Related Art
A common viewpoint held by a significant segment of the environmental community is that carbon dioxide released into the air plays a major role in global climate change. Thus, global climate change initiatives such as the Kyoto Protocol have identified the curtailment of carbon dioxide releases from fossil fuel combustion and other point sources as a primary means of reducing global climate change. Extensive programs already in place are beginning to demonstrate the economic and technical feasibility of sequestering carbon dioxide by approaches such as injection in underground reservoirs (Bergman et al., 1996) and disposal in the deep ocean (Fujioka et al., 1996).
One method of curtailing carbon dioxide releases in the industrial arena involves removing carbon dioxide from combustion gases and other gases. Carbon dioxide is emitted in large quantities from fuel combustion by mobile and stationary sources. Carbon dioxide capture/sequestration will be most effective if applied to large stationary sources. The largest single sources of carbon dioxide are conventional coal-fired power plants. These sources represent 30 to 40% of the carbon dioxide emissions in the United States. Technology developed for such sources should also be applicable to CO2 capture from gas and oil fired boilers, combined cycle power plants, coal gasification, and hydrogen plants. Absorption/stripping is primarily a tail-end technology and is therefore suitable for both existing and new boilers. Specifically, it can be used with existing coal-fired boilers, especially if they already have scrubbers for SO2-abatement.
The use of absorption and stripping processes with aqueous solvents such as alkanolamines and promoted potassium carbonate is a known, effective technology for the removal and capture of carbon dioxide from flue gas, natural gas, hydrogen, synthesis gas, and other gases. U.S. Pat. Nos. 4,477,419 and 4,152,217, each of which is incorporated herein by reference, describe aspects of this technology. Alkanolamine absorption/stripping is one proven and effective technology for carbon dioxide capture from gas. The first generation of this technology uses aqueous solutions of monoethanolamine (MEA). Advances in this technology have provided other alkanolamine solvents for carbon dioxide treating in various industries. Monoethanolamine (MEA), diethanolamine (DEA), and the hindered amine AMP are used alone in an aqueous solution. Typical solvent blends include a methyldiethanolamine (MDEA) solution promoted by piperazine or other secondary amines. Also, potassium carbonate solvents are commonly promoted by DEA or other reactive amines.
Gas absorption is a process in which soluble components of a gas mixture are dissolved in a liquid. Stripping is essentially the inverse of absorption, as it involves the transfer of volatile components from a liquid mixture into a gas. In a typical carbon dioxide removal process, absorption is used to remove carbon dioxide from a combustion gas, and stripping is subsequently used to regenerate the solvent and capture the carbon dioxide contained in the solvent. Once carbon dioxide is removed from combustion gases and other gases, it can be captured and compressed for use in a number of applications, including sequestration, production of methanol, and tertiary oil recovery.
The conventional method of using absorption/stripping processes to remove carbon dioxide from gaseous streams is described in U.S. Pat. No. 4,384,875, which is incorporated herein by reference. In the absorption stage, the gas to be treated, containing the carbon dioxide to be removed, is placed in contact, in an absorption column, with the chosen absorbent under conditions of pressure and temperature such that the absorbent solution removes virtually all the carbon dioxide. The purified gas emerges at the top of the absorption column and, if necessary, it is then directed towards a scrubber employing sodium hydroxide, in which the last traces of carbon dioxide are removed. At the bottom of the absorption column, the absorbent solution containing carbon dioxide (also called “rich solvent”) is drawn off and subjected to a stripping process to free it of the carbon dioxide and regenerate its absorbent properties.
To effect the regeneration of the absorbent solution, the rich solvent drawn off from the bottom of the absorption column is introduced into the upper half of a stripping column, and the rich solvent is maintained at its boiling point under pressure in this column. The heat necessary for maintaining the boiling point is furnished by reboiling the absorbent solution contained in the stripping column. The reboiling process is effectuated by indirect heat exchange between part of the solution to be regenerated located in the lower half of the stripping column and a hot fluid at appropriate temperature, generally saturated water vapor. In the course of regeneration, the carbon dioxide contained in the rich solvent to be regenerated maintained at its boiling point is released and stripped by the vapors of the absorbent solution. Vapor containing the stripped carbon dioxide emerges at the top of the stripping column and is passed through a condenser system which returns to the stripping column the liquid phase resulting from the condensation of the vapors of the absorbent solution which pass out of the stripping column with the gaseous carbon dioxide. At the bottom of the stripping column, the hot regenerated absorbent solution (also called “lean solvent”) is drawn off and recycled to the absorption column after having used part of the heat content of the solution to heat, by indirect heat exchange, the rich solvent to be regenerated, before its introduction into the stripping column.
In simple absorption/stripping as it is typically practiced in the field, aqueous rich solvent is regenerated at 100-120° C. in a simple, countercurrent, reboiled stripper operated at a single pressure, which is usually 1-2 atm. The rich solvent feed is preheated by cross-exchange with hot lean solvent product to within 5-30° C. of the stripper bottoms. The overhead vapor is cooled to condense water, which is returned as reflux to the countercurrent stripper. When used for carbon dioxide sequestration and other applications, the product carbon dioxide is compressed to 100-150 atm.
A major challenge facing the implementation of aqueous absorption/stripping on a large scale for CO2 capture is the high capital cost of columns, pumps and exchangers and initial solvent and operating cost (reboiler duty, pump circulation rate, solvent make-up) of the technology. If applied to a coal-fired power plant, this may reduce the power output by 20-40% (Rochelle, 2003). Current efforts to reduce the capital and operating cost include the development of alternative solvents to the industrial state-of-the art, 7 m (30-wt %) MEA, the use of innovative process configurations, flowsheet optimization, and energy integration with other sections of the power plant. Alternative solvents should provide equivalent or greater CO2 absorption rates than MEA, adequate capacity for CO2, and reduced cost of regeneration. The important alternative solvents are promoted K2CO3 (Cullinane et. al, 2002-05), promoted MEA (Dang; Okoye), promoted tertiary amines (Idem et al.; Aroonwilas et al., 2006; Bishnoi), and mildly hindered amines (Mitsubishi). Fluor has developed an improved MEA process (MEA with some corrosion inhibitors). Mitsubishi Heavy Industries (MHI) and Kansai Electric Power Co. Inc. have developed the solvent KS-1 (Mimura et al.; Yagi et al.). The Research Institute of Innovation Technology for the Earth (RITE) has developed some solvents (Shimizu et al.) and Svendsen and co-workers (Ma'mum et al.; Hoff et al.) have screened other solvents. Amino acid salts have been tested for gas absorption/membrane hybrid applications at TNO, Netherlands (Versteeg et al.; Feron et al.). The potential use of ionic liquids for CO2 capture has also been evaluated (Bates et al.; Dixon et al.).
Some alternative process configurations that have been proposed to reduce capital and operating costs of the CO2 capture process include the use of multiple absorber feeds and split flow for the gas sweetening industry (Bullin et al.; Polasek et al.). The performance and cost structure of the split flow configuration has been evaluated (Aroonwilas et al., 2004, 2006), as have vacuum and multipressure configurations (Oyenekan et al., 2005, 2006) and multipressure stripping with vapor recompression (Jassim et al.). Others have proposed other more complex configurations to reduce energy requirement for CO2 removal (Leites et al.).
In light of the above, it would be advantageous to provide for technology in which carbon dioxide and other acid gases can be removed from combustion gases and other gases by an absorption/stripping process that is significantly more energy efficient than the processes currently practiced.
Any problems or shortcomings enumerated in the foregoing are not intended to be exhaustive but rather are among many that tend to impair the effectiveness of previously known techniques. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that apparatus and methods appearing in the art have not been altogether satisfactory and that a need exists for the techniques disclosed herein.